To improve the performance of mass ejection drive systems, it is sometimes desirable to rotate the drive system about its principal axis. Conventional structures for producing rotation include fluted nozzles and ablative vanes.
Fluted nozzles are generally fixed structures that are at least partially disposed within the ejecta stream of the mass ejection drive system. These structures impart torque and affect rotation of the drive system as a result of their incidence with matter in the ejecta stream. However, because fluted nozzles are fixed and considerably durable, they generally affect rotation for substantially the entire duration of mass ejection. This poses a number of problems for drive systems configured for use with launch tubes. For mass ejection drive systems configured for deployment from a launch tube, the duration of mass ejection is likely to exceed the time that it takes for the device to exit the launch tube. As a consequence, fluted nozzles will typically continue to impart rotation to the drive system following exit from the launch tube. Additionally, because the duration of mass ejection is likely to exceed the time that it takes to exit the launch tube, maximum rotation will generally not be achieved upon exit from the launch tube, but at some subsequent point in time. These issues may operate to complicate targeting calculations and reduce predictability of the drive system with respect to targeting.
Some of the problems associated with fluted nozzles may be ameliorated with the use of ablative vanes. In general, ablative vanes may be suitably configured to provide the drive system with periods of mass ejection in which there is substantially no rotation imparted to the system. Ablative vanes may also be suitably configured to produce higher rates of accelerated rotation than fluted nozzles. However, problems with ablative vanes include irregular and unpredictable erosion of the vanes. Moreover, for drive systems configured for deployment from a launch tube, ablative vanes have a tendency to foul the launch tube by virtue of their substantially direct contact with launch tube surfaces.